polkadot-sdk-docs: Use command_macro! (#6624)

# Description

**Understood assignment:**
Initial assignment description is in #6194.
In order to Simplify the display of commands and ensure they are tested
for chain spec builder's `polkadot-sdk` reference docs, find every
occurrence of `#[docify::export]` where `process:Command` is used, and
replace the use of `process:Command` by `run_cmd!` from the `cmd_lib
crate`.

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Co-authored-by: Iulian Barbu <[email protected]>